<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <script type="text/javascript" src="./script/jquery-3.7.1.min.js"></script>
  <script type="text/javascript">
    window.onload=function (){
      var username = document.getElementById("username");
      alert("dom的username "+username.value)

      var $username = $(username);
      alert("jquery的username "+$username.val())
    }
  </script>
</head>
<body>
输入<input type="text" id="username" name="username"value="gzggzgz">
</body>
</html>